Generally, the structure of the two axis (XY, for example) stage for microscopes has a reference base, an X table built on the base and movable in an X direction, and a Y table mounted on the X table and movable in Y direction perpendicular to the X direction. For the XY stage, rotary motors and feed screws are often used as actuators, as shown in JP Patent Publication (Kokai) No. 7-142558 A (1995). The actuator include similar feed mechanisms for both X and Y tables. Each feed mechanism consists of a nut unit that is fixed on a moving side, a feed screw that engages with the nut unit on the moving side and is rotatably and axially supported in the moving direction, and a motor to rotate the feed screw.
When using the XY stage in a chamber in which a specific atmosphere or vacuum is maintained, it is desirable that the motor be positioned outside the chamber for reasons of discharge, longer life, and maintenance. However, in the XY stage, it is difficult to position the motor outside the chamber because the Y table feed mechanism moves in the X direction along with the movement of the X table. Therefore, conventionally, as shown in FIG. 6, the movement of the Y table 31 in the Y direction is made possible by pushing and pulling a guide rail 37 positioned at the edge of the Y table 31 and slidable in the X direction, using a Y-drive shaft 36 connected with a Y feed screw 35 in a sub-chamber 33 disposed outside the chamber 32. In this way, the lateral difference between the Y table 31 and the Y feed screw 35 caused by the movement of the X table 30 can be eliminated. Numeral 29 designates a base, numeral 34 an X feed screw, numeral 38 an X drive motor, and numeral 39 a Y drive motor.
JP Patent Publication (Kokai) No. 7-142558 A (1995) discloses an XY stage comprising a stacked arrangement of an X-direction transport mechanism and a Y-direction transport mechanism.
In the XY stage disclosed in JP Patent Publication (Kokai) No. 7-142558 A (1995), the total height of the stage increases because the two tables, each with a feed mechanism for transporting in one axial direction, are stacked such that they move at right angles to each other. Further, the gravitational center of the entire stage is high because the feed mechanism for driving the Y table (upper table) is mounted on the X table, thereby reducing the mechanical resonance frequency of the stage. Also, there is a problem that the X table feed mechanism requires a powerful motor for fast movement because of the increased weight to be moved.
When using the XY stage in a chamber in which a specific atmosphere or vacuum is maintained, the method shown in FIG. 6, in which the sub-chamber 33 is provided outside the chamber 32, leads to an increased floor-projected area of the entire apparatus. Moreover, the positional precision of the Y table 31 could be affected by the change of thrust point in the Y table 31 in accordance with the movement of the X table 30.